Description: Chlorofluorocarbon (component CFC-11) and hydrographic data from 1997, 1999, and 2001 are presented to track the large-scale spreading of the Upper Labrador Sea Water (ULSW) in the subpolar gyre of the North Atlantic Ocean. ULSW is CFC rich and comparatively low in salinity. It is located on top of the denser “classical” Labrador Sea Water (LSW), defined in the density range σΘ = 27.68–27.74 kg m−3. It follows spreading pathways similar to LSW and has entered the eastern North Atlantic. Despite data gaps, the CFC-11 inventories of ULSW in the subpolar North Atlantic (40°–65°N) could be estimated within 11%. The inventory increased from 6.0 ± 0.6 million moles in 1997 to 8.1 ± 0.6 million moles in 1999 and to 9.5 ± 0.6 million moles in 2001. CFC-11 inventory estimates were used to determine ULSW formation rates for different periods. For 1970–97, the mean formation rate resulted in 3.2–3.3 Sv (Sv ≡ 106 m3 s−1). To obtain this estimate, 5.0 million moles of CFC-11 located in 1997 in the ULSW in the subtropical/tropical Atlantic were added to the inventory of the subpolar North Atlantic. An estimate of the mean combined ULSW/LSW formation rate for the same period gave 7.6–8.9 Sv. For the years 1998–99, the ULSW formation rate solely based on the subpolar North Atlantic CFC-11 inventories yielded 6.9–9.2 Sv. At this time, the lack of classical LSW formation was almost compensated for by the strongly pronounced ULSW formation. Indications are presented that the convection area needed in 1998–99 to form this amount of ULSW exceeded the available area in the Labrador Sea. The Irminger Sea might be considered as an additional region favoring ULSW formation. In 2000–01, ULSW formation weakened to 3.3–4.7 Sv. Time series of layer thickness based on historical data indicate that there exists considerable variability of ULSW and classical LSW formation on decadal scales.

Description: Diese Broschüre gibt einen verständlichen Überblick über den wissenschaftlichen Kenntnisstand zur Golfstromzirkulation. Forscherinnen und Forscher ordnen verwirrende und oft widersprüchliche Informationen ein, die in der Öffentlichkeit diskutiert werden. Die Broschüre bietet Orientierung über plausible Zukunftsszenarien und relevante Forschungsfragen.

Description: Auf 32 Seiten geben die beiden Wissenschaftsverbände einen verständlichen Überblick zum Meeresspiegelanstieg. 14 Wissenschaftlerinnen und Wissenschaftler unterschiedlicher Forschungseinrichtungen ordnen gemeinsam die Informationen ein, die immer wieder in der Öffentlichkeit diskutiert werden, und erklären die wichtigsten Zusammenhänge und zugrundeliegenden Prozesse in klaren Worten. Damit bietet die Broschüre Orientierung in Bezug auf plausible Zukunftsszenarien, und hilft, die Risiken besser einzuschätzen. Zusätzlich erläutern die Forschenden die Situation an den deutschen Küsten, denn Klimawandel und Meeresspiegelanstieg betreffen auch die Nord- und Ostsee.

Description: Die Meeresströmungen im Atlantik spielen für unser Klima eine wichtige Rolle. Klimamodelle zeigen, dass bei weiter steigenden Treibhausgas-Emissionen die Stärke der Strömungen abnimmt und sich ihr Verlauf ändert. Dies hat weitreichende Folgen für die regionale Erwärmung, Niederschläge, Meeresspiegel, Landwirtschaft und Fischerei auch in Deutschland. Deshalb haben die führenden deutschen Meeresforschungsinstitute Langzeitbeobachtungen der Meeresströmungen an Schlüsselstellen im Atlantik installiert. Durch sie kennen wir nun die Strömungsstärken und ihre Schwankungen über Zeiträume von Stunden bis Jahrzehnte und können Klimatrends frühzeitig erkennen. Die Messungen haben auch gezeigt, dass selbst in den aktuellsten Klimamodellen noch immer große Unterschiede zwischen den simulierten und den beobachteten Strömungen bestehen und auch die vorhersagte Abschwächung der Strömungen bis ins Jahr 2100 in den Modellen unterschiedlich ausfällt. Um die Ergebnisse der Klimamodelle auch in Zukunft durch Beobachtungen bewerten zu können, müssen die Langzeitbeobachtungen der Atlantikzirkulation aufrechterhalten werden. Diese Broschüre knüpft an die bereits erschienenen Bände „Zukunft der Golfstromzirkulation“ (2016) und „Zukunft der Meeresspiegel“ (2019) an, in die ebenfalls Resultate aus den Langzeit-Beobachtungssystemen eingeflossen sind.

Description: Bottom water temperatures in the central Greenland Sea have been increasing for the last two decades. The warming is most likely related to the absence of deep convective mixing, which cools and freshens the deep water. However, recent observations confirm a slow and steady increase of anthropogenic tracers such as chlorofluorocarbons (CFCs). This points to some amount of bottom water “ventilation” in the absence of deep convective mixing and poses a challenge to our understanding of deep water renewal. One explanation for the observed trends in both temperature and CFCs is significant vertical mixing. The basin-averaged diapycnal diffusivity, required to explain both trends, kυ,av 2–3 (×10−3 m2 s−1), is very unlikely to occur in the interior of the ocean. However, a diffusivity of kυ,bbl 10−2 m2 s−1 within a 150-m thick bottom boundary layer would be sufficient to explain the deep tracer increase. The implications of a secondary circulation driven by such large boundary layer mixing are discussed.

Description: In 1997, a unique hydrographic and chlorofluorocarbon (CFC: component CFC-11) dataset was obtained in the subpolar North Atlantic. To estimate the synopticity of the 1997 data, the recent temporal evolution of the CFC and Labrador Sea Water (LSW) thickness fields are examined. In the western Atlantic north of 50°N, the LSW thickness decreased considerably from 1994–97, while the mean CFC concentrations did not change much. South of 50°N and in the eastern Atlantic, the CFC concentration increased with little or no change in the LSW thickness. On shorter timescales, local anomalies due to the presence of eddies are observed, but for space scales larger than the eddies the dataset can be treated as being synoptic over the 1997 observation period. The spreading of LSW in the subpolar North Atlantic is described in detail using gridded CFC and LSW thickness fields combined with Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float trajectories. The gridded fields are also used to calculate the CFC-11 inventory in the LSW from 40° to 65°N, and from 10° to 60°W. In total, 2300 ± 250 tons of CFC-11 (equivalent to 16.6 million moles) were brought into the LSW by deep convection. In 1997, 28% of the inventory was still found in the Labrador Sea west of 45°W and 31% of the inventory was located in the eastern Atlantic. The CFC inventory in the LSW was used to estimate the lower limits of LSW formation rates. At a constant formation rate, a value of 4.4–5.6 Sv (Sv ≡ 106 m3 s−1) is obtained. If the denser modes of LSW are ventilated only in periods with intense convection, the minimum formation rate of LSW in 1988–94 is 8.1–10.8 Sv, and 1.8–2.4 Sv in 1995–97